linux-kernel - [PATCH] pwm: mxs: fix period divider computation

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Message-ID: <1424264794-12167-1-git-send-email-ghug@induct.be>
Date:	Wed, 18 Feb 2015 14:06:34 +0100
From:	Gaetan Hug <ghug@...uct.be>
To:	<thierry.reding@...il.com>
CC:	<linux-pwm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<shawn.guo@...aro.org>, <fabio.estevam@...escale.com>,
	Gaetan Hug <ghug@...uct.be>
Subject: [PATCH] pwm: mxs: fix period divider computation

The driver computes which clock divider it sould be using from the
requested period. This computation assumes that the link between the
register value and the actual divider value is raising 2 to the power of
the registry value.

    div = 1 << regvalue

This is true only for the first 5 values out of 8. Next values are 64,
256 and, 1024 - instead of 32, 64, 128.
This affects only the users requesting a period > 0.04369s.

Replace the computation with a look-up table.
---
 drivers/pwm/pwm-mxs.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-mxs.c b/drivers/pwm/pwm-mxs.c
index f75ecb0..c65e183 100644
--- a/drivers/pwm/pwm-mxs.c
+++ b/drivers/pwm/pwm-mxs.c
@@ -35,6 +35,8 @@
 #define  PERIOD_CDIV(div)	(((div) & 0x7) << 20)
 #define  PERIOD_CDIV_MAX	8
 
+static unsigned const int cdiv[PERIOD_CDIV_MAX] = {1, 2, 4, 8, 16, 64, 256, 1024};
+
 struct mxs_pwm_chip {
 	struct pwm_chip chip;
 	struct clk *clk;
@@ -54,13 +56,13 @@ static int mxs_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	rate = clk_get_rate(mxs->clk);
 	while (1) {
-		c = rate / (1 << div);
+		c = rate / cdiv[div];
 		c = c * period_ns;
 		do_div(c, 1000000000);
 		if (c < PERIOD_PERIOD_MAX)
 			break;
 		div++;
-		if (div > PERIOD_CDIV_MAX)
+		if (div >= PERIOD_CDIV_MAX)
 			return -EINVAL;
 	}
 
-- 
1.7.10.4

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